Is a switch from Java to Python worth it?
While dyedgreen is right in some respects, I don't agree entirely with that sentiment. Sure, you can theoretically use any language as long as you know the maths and understand the concepts inside and out whilst having some applicable knowledge. 

However, I don't believe if you are starting from scratch, you should learn to develop models in Java. While the underlying material is the same, all you are doing is wasting time IMO learning a language that is rarely used in the field and lacks sufficient support by most modern ML frameworks. Also, while this is rather subjective, python feels better when used as a data science language. Working with data especially is where that difference is clear to me. 

That being said, this could all change, and Java could become the ML language of choice(extremely unlikely).